Simple Web Server Availability Monitoring with cron, bash, and wget

I recently needed a really, really simple remote monitoring agent to keep track of server availability. Actually, I didn’t even need a record of uptime or anything; I merely needed to be alerted if any of a few web servers had gone down unexpectedly. So, I wrote one.

I know there are lots of other solutions for this problem, many of which are free. You can do this with Nagios, for example. But I don’t have an existing Nagios installation already setup, and I didn’t want to deploy monstrous beast of a platform of which I needed only 0.1% of the capabilities. There are also many web-based services that will do this for you, but most of these have a pretty large “check” interval, or else they cost money. I was feeling both cheap and picky, and I knew everything I needed to do could be accomplished using cron, bash, and wget. It works great, so I wanted to share what I came up with.

Here is a basic feature list:

• Requires bash, wget, and working “mail” command (sendmail, exim, postfix, etc.)
• Monitors any HTTP or HTTPS URL, checking for “200” status returned
• Checks request fulfillment time for slow responses, not just UP/DOWN
• Sends email notification on state change (UP, SLOW, or DOWN)
• Customizable To/From notification settings
• Customizable SLOW latency threshold
• Tracks previous state change and last update to avoid multiple notifications
• Uses single text file for data storage, no DB necessary

It’s pretty basic, but it works. Just save this script somewhere, modify the HOSTS and other settings to taste, and add a cron job definition to run it every five minutes or so. No Nagios or 3rd party solutions required. If this is what you’re looking for, then…awesome!

Sample cron job definition

*/5 * * * * root /home/username/sitemonitor.sh

sitemonitor.sh script source

#!/bin/bash

# Simple HTTP/S availability notifications script v0.1
# March 20, 2012 by Jeff Rowberg - http://www.sectorfej.net

HOSTS=( \
    "http://www.yahoo.com" \
    "https://www.google.com" \
    "http://www.amazon.com" \
    )

NOTIFY_FROM_EMAIL="Site Monitoring <monitoring@example.com>"
NOTIFY_TO_EMAIL="Server Admin <admin@example.com>"

STATUS_FILE="/home/username/sitemonitor.status"

SLOW_THRESHOLD=10
OK_STATUSES=( "200" )

################################################################
#        NO MORE USERMOD STUFF BELOW THIS, MOST LIKELY         #
################################################################

# thanks to stackoverflow!
# stackoverflow.com/questions/3685970/bash-check-if-an-array-contains-a-value
function contains() {
    local n=$#
    local value=${!n}
    for ((i=1; i < $#; i++)) {
        if [ "${!i}" == "${value}" ]; then
            echo "y"
            return 0
        fi
    }
    echo "n"
    return 1
}

rm -f /tmp/sitemonitor.status.tmp
for HOST in "${HOSTS[@]}"
do
    START=$(date +%s)
    RESPONSE=`wget $HOST --no-check-certificate -S -q -O - 2>&1 | \
                  awk '/^  HTTP/{print \$2}'`
    END=$(date +%s)
    DIFF=$(( $END - $START ))
    if [ -z "$RESPONSE" ]; then
        RESPONSE="0"
    fi
    if [ $(contains "${OK_STATUSES[@]}" "$RESPONSE") == "y" ]; then
        if [ "$DIFF" -lt "$SLOW_THRESHOLD" ]; then
            STATUS="UP"
        else
            STATUS="SLOW"
        fi
    else
        STATUS="DOWN"
    fi
    touch $STATUS_FILE
    STATUS_LINE=`grep $HOST $STATUS_FILE`
    STATUS_PARTS=($(echo $STATUS_LINE | tr " " "\n"))
    CHANGED=${STATUS_PARTS[2]}
    if [ "$STATUS" != "${STATUS_PARTS[5]}" ]; then
        #if [ -e "${STATUS_PARTS[5]}" ] || [ "$STATUS" != "UP" ]; then
            if [ -z "${STATUS_PARTS[5]}" ]; then
                STATUS_PARTS[5]="No record"
            fi
            TIME=`date -d @$END`
            echo "Time: $TIME" > /tmp/sitemonitor.email.tmp
            echo "Host: $HOST" >> /tmp/sitemonitor.email.tmp
            echo "Status: $STATUS" >> /tmp/sitemonitor.email.tmp
            echo "Latency: $DIFF sec" >> /tmp/sitemonitor.email.tmp
            echo "Previous status: ${STATUS_PARTS[5]}" >> /tmp/sitemonitor.email.tmp
            if [ -z "${STATUS_PARTS[2]}" ]; then
                TIME="No record"
            else
                TIME=`date -d @${STATUS_PARTS[2]}`
            fi
            echo "Previous change: $TIME" >> /tmp/sitemonitor.email.tmp
            `mail -a "From: $NOTIFY_FROM_EMAIL" \
                  -s "SiteMonitor Notification: $HOST is $STATUS" \
                  "$NOTIFY_TO_EMAIL" < /tmp/sitemonitor.email.tmp`
            rm -f /tmp/sitemonitor.email.tmp
        #else
             # first report, but host is up, so no need to notify
        #fi
        CHANGED="$END"
    fi
    echo $HOST $RESPONSE $CHANGED $END $DIFF $STATUS >> /tmp/sitemonitor.status.tmp
done

mv /tmp/sitemonitor.status.tmp $STATUS_FILE

Sample notification email

Time: Fri Mar 23 17:20:02 UTC 2012
Host: http://www.example.com
Status: UP
Latency: 0 sec
Previous status: DOWN
Previous change: Fri Mar 23 15:05:20 UTC 2012

Pretty simple stuff, but it totally gets the job done, and you can have it check on any interval and as many servers as you want! That’s hard to beat at this level of simplicity and freedom.